Piston pumps are well known in which a piston is reciprocally movable in a chamber for pumping fluids. It is well known to provide a radially outwardly extending vane on a piston to extend outwardly and engage an internal chamber wall so as to guide the piston in coaxial alignment while sliding within the chamber. The present inventor has appreciated that previously known vanes suffer the disadvantage that they must avoid sealing discs provided in the piston and typically cannot extend axially of the piston for the entire length of the piston.